Underbelly style: Mounts under the frame of the trailer at the place it will be used.
Portable style: Carrier and platform are mounted under the trailer’s frame, enabling the driver to move the platform to different doors.
In floor (tunnel) style: Installed at the trailer factory in the trailer’s floor.
Whether made of aluminum or steel, be sure when you're ordering equipment to specify that you want open surface materials that provide the best friction and keep walk surfaces free of debris, such as snow, rain, ice, mud, etc. Also make sure that the published load ratings are given with a safety factor and not merely the ultimate break strength.